In most video displays in which the display is changeable, the picture pixels are arranged in rows and columns. The invention is applicable to any display of that kind whether it be a cathode ray tube display or a newer grid LED or liquid crystal or color grid display, or any other including the old electric light bulb displays. Currently, the advantages of the invention are most apparent when it is applied to cathode ray color tube displays.
A pixel is defined as the smallest area of a digital display screen all of which must have the same color where the term "color" means color value or hue or shade. The term implies that the color of an individual pixel may and can have a color different from that of any pixel adjacent to it in a display.
In most cases the color or range of colors that can be displayed on the screen of a given display apparatus is determined by that apparatus. Control of what colors are displayed within that range can be accomplished with a wide variety of digital devices the more practical forms of which are electronic in nature. The color, cathode ray picture tube creates images by sweeping a beam across the face of the tube where it excites chemicals on the face to visible phospheresence. The beam sweeps across the tube face again and again on adjacent parallel straight paths to form a raster over the entire screen. In one convention, the raster is formed by 528 horizontal, closely spaced parallel straight lines, and the raster is formed thirty times per second. On such a screen a pixel is an area formed by a length of at least one line of the raster. Because the rate at which the beam moves across the screen is usually uniform, the time to trace a line is fixed.
Accordingly, the width of a pixel can be defined as a time--some fraction of the time required to sweep a line. The height of a pixel can be defined in terms of number of sweep lines. In one standard, the display is divided into pixels such that each pixel is one line high, and such that there are 536 across the width of the screen. In that standard, a "character" is eight pixels high and eight pixels long.
To create an image in different hues on the screen of a color picture tube, it is required to energize the three guns of the tube in different relative degree. To color one pixel of the screen requires that the tube guns be energized to produce the desired color each time that the beams of those guns trace through that one pixel for the time that the beam is traversing the width of the pixel. To do that requires timing information for use in ensuring that the selected pixel is the one that is colored, and it requires color information to ensure that the pixel is given the selected color. Color information must be supplied for each pixel. When it must be supplied is a matter of timing, controlled by the clock which controls raster generation, or by a separate clock that is synchronized with the raster control clock.
In the case of a black and white display, only one bit of information is required for each pixel to specify whether it is to be colored black or white. Three bits of information will specify any of eight colors and that number is common in many applications.
In some applications, all of the color information bits are generated by a computer. That might be true in the case of graphical solution of mathematical equations. In other applications, as for example in interactive computerized teaching and in video games, much of the color information is stored in memory devices to be fetched by a computer, or otherwise. In many cases, for example in the case of video games that involve many different characters or playing pieces, enough information must be stored so that the cost of memory devices is relatively high. In the past, the only relief from cost was to sacrifice resolution by reducing the number of pixels or, in some cases, by reducing the number of characters.